Sharif Digital Repository

The interaction of the vortices and helicopter’s blades is the most significant source of generating the main rotor’s noise, especially in landing and rising. The kind of this noise is broadband and it is produced because of establishing the vortices behind the forward blade, their interaction with backward blade, changing their structures and creating turbulency loading. The known method for modeling of the interaction of the vortices structures and solid surfaces is the simple compounding the rod and airfoil. In this project, by using this method, the noise of the interaction of vortex and blade is modeled in the Reynolds number of 48000 with LES and FW-H Analogy as a hibrid method in... 

Electrospray is one of the liquid spraying methods. This study has an insight on Electrospray phenomenon and its effective parameters. First, there is a short introduction of phenomenon, then, how it works and effective factors related to the procedure are described. All modes, the forces and the situation to establish has been investigated for ethanol sprayed from hemispherical nozzle. Suitable configuration is introduced and for different flow rates and voltages the phenomenon is studied. One of the innovations of this study is the imaging technique used to captured high-quality images. In fact, captured images by means of high-power light-emitting diode (LED) synchronized with digital... 

One of the extensively used, yet complex, structures in the industry is pre-twisted Micro/Nano beams. Studying their mechanical behavior helps to have a broader view of them. In this present study, explores and analyzes the behavior of a pre-twisted Micro/Nano beam with a quadrangular/rectangular cross-section using the strain gradient theory and modified couple stress theory. Using the calculus of variations and the Hamiltonian principle the elastodynamics governing partial differential equations of transverse deflection of the pre-twisted Micro/Nano beam with hinged-hinged boundary conditions are derived. Then the mechanical behavior of the pre-twisted Micro/Nano beam in static mode and... 

The main purpose of the current study is to control the boundary layer of the flow passing over the wing and the lift force enhancement. In this research has been tried to introduce a new generation of passive flow control and use bumpy surface structures to control the separated flow and increase the aerodynamic efficiency. Therefore, the wing with the hawk bird’s airfoil section has been designated. Then, assuming a viscous, incompressible flow and a low Reynolds number range (about 2 × 105), the flow formation and boundary layer development on this wing has been investigated. Subsequently, bump structures have been settled on the upper surface of this wing. The numerical simulations have... 

The emission of carbon dioxide gas is one of the environmental problems in the world and the elimination of CO2 receives more attention by researchers day to day. In this study ,adsorption of carbon dioxide on fullerenes adsorbents such as C26, C32, C36, C38and C44 in the absence and in the presence of electric field and also on nitrogen-doped fullerenes such as C25N, C31N, C35N, C37N and C43N in the absence and in the presence of electric field were investigated. Calculations were performed using density functional theory at the level of B3LYP/6-311++G(d,p). The computations represent that doping with nitrogen has not effect considerably on adsorption of carbon dioxide and applied electric... 

In recent years, with the advances in powerful neural networks, techniques called transfer learning and self-supervised methods have become more popular. Also, attention mechanism modules have naturally provided the possibility of pre-training with a self-supervised method. Embedding these modules in recent transformer-based architectures has improved image and text tasks. In this study, a way to enhance the visual input vectors of the multimode Bert model based on transformer architecture is introduced. Also, an effective and interpretable structure of a combined convolution and transformer method is presented, which is pre-trained on data from this field before realignment for a medical... 

Gas turbine efficiency will increase with rising the temperature of inlet gas. However, there are some restrictions for rising the temperature of inlet gas. Film cooling is one of the best methods to eliminate these limitations. The geometry of coolant jet hole is one of the most effective parameters in effectiveness and uniformity of film cooling. In this experimental investigation, the effects of five different coolant jet holes are evaluated. Blowing ratios of 0.5, 1 and 1.5 and density ratio of 1 were selected to perform this study. The five models for coolant hole are: square model, CTJ model, 3-circle model, semicircle horseshoe model and parabolic horseshoe model. The injection angle... 

Pipe freezing has become well established in industry as a method of short-termisolation in order to carry out maintenance or repairs. It is often necessary to repair system components without plant shutdown or drainage of system having many piping structures which may have hazardous or expensive fluid. Therefore, a series of tests has been arranged to investigate the ice plug formation in two standard pipes with different diameter size. The freeze zone on the pipe has been created by means of an aluminum jacket which liquid nitrogen flows in it. In the tests the freezing time, amount of liquid nitrogen usage andthe pressure tolerance of ice plug have been determined. Moreover, the numerical... 

The present thesis aims at investigating the dynamic interaction of slosh and aeroelasticity in the fuel contained aero vehicles. The main approach in this research is to develop reduced order models for description of the coupled system. In this regard, boundary element method is used to develop slosh dynamic model. Axisymmetric boundary element method with non-symmetric boundary conditions is used to develop slosh dynamic model for axisymmetric containers. Zoning method is used to develop slosh dynamic model for the multi-baffled tanks and based on it, slosh equivalent mechanical model is developed for multi-baffled tanks. Finite element method along with modal technique is used to develop... 

In recent years, the area of research on micro flying robots has been developed due to the development in micro manufacturing technologies. Due to extra advantages of these flying robots, they are implemented in military and civilian missions such as identifying, espionage, aerial relays, traffic surveillance, rescue, aid, etc. Ability to fly in specified areas, without human injuries, low manufacturing and energy costs, easy and fast implementation, Easy Maintenance, etc, which are some advantages of these robots. We can control these robots manually by remote controllers (RC) or by Auto guidance systems. On the other hand, these robots are used in research laboratories for testing new... 

Range Profile (RP) is known as one of the most important tools for radar target recognition. The main problem with range profile for radar target recognition is its sensitivity to aspect angle. To overcome this problem, range profiles are assumed to have the same statistical characteristics in small frames of aspect angles or context dependent models such as HMM may be used. All methods presented, seems to have some shortcomings to offer a model based on physical circumstances of target maneuver. For example in such models, consecutive samples of RPs in an aspect frame are assumed to be statistically independent with the same distribution (IID). Here we propose dynamic system (DS) for... 

There are numerous ways available to achieve low temperatures that cryocoolers might be among the most important of them. In the meantime, the pulse tube refrigerator has a special place due to its simple geometry and not be using any moving parts. This simple property make the pulse tube refrigerator one of the most popular methods to achieve low temperatures. Nowadays, a pulse tube refrigerator is used for cooling in the range of 4 to 123 Kelvin in two form of one-stage and two-stage and up to an approximate power of 1 KW. Today, due to the temperature and the amount of power required for cooling, as well as the time it takes to reach that temperature, this chiller is used in medical,... 

This work deals with the computational investigation of film cooling technique, which is one the best practical way to protect gas turbine components form high thermal loads. In this regards, previous works are extensively reviewed and most important effective parameters are classified into three general categories, as geometrical parameters, flow characteristics, and physical surface factors. Each of these categories is then divided into subcategories and more details studies of each are performed. Then, a novel near-wall flow control technique of using staggered arrangement of small injection ports near a film cooling hole (combined-triple-jet; CTJ) is introduced. The fluid injected from... 

Smoothed particle hydrodynamics (SPH) is a meshfree method, based on Lagrangian formulation of Navier-Stokes equations, in which fluid is discretized to particles. This method is suitable and gives much better results for problems in which other methods have severe difficulties, (e.g., tracking of particles, moving boundaries, and problems with large deformations and, physical discontinuity). No need to follow a regular geometric structure in solution process and simple adaptation and coupling with methods such as finite element, are the positive characteristics of this numerical method. In this work, a systematic study of the processes, which are part of a membrane failure in a... 

Boundary layer control has long been and still is considered to reduce drag. In this research, the effect of quasi-spherical structures on the control of boundary layer is investigated. First, flow over a NACA0015 airfoil was investigated. Then, flow over the same airfoil with quasi-spherical bumps on its upper surface was numerically simulated and the result of these two situations were compared. The dimensions and numbers of the bumps in longitudinal and lateral directions (different case studies) were chosen using trial and error. The three-dimensional, incompressible, and turbulent flow governing equations were numerically solved, using FVM and the OpenFOAM software. To get preliminary... 

The limitations of metals in tolerating thermal stresses is one of the main obstacles in increasing temperature of combustion products. In addition, strong desires to use higher temperatures than are allowed for metals, have led to use of different cooling methods for protecting surfaces adjacent to hot gases. Previous valid studies show that more than 25% of research in the field of gas turbine is related to their blades cooling. On the other hand, one of the very important methods of cooling such surfaces is film cooling. The results of this and previous researches conducted by the team of this thesis’ supervisors can for instance help designers to predict more suitable positions for... 

During the last few years, the study of urban environment structures is considered as a research field of interest in remote sensing. In satellite observations of the earth's surface, continuous imaging in terms of time and space has caused the remote sensing technique to be proposed as a useful and efficient tool for the analysis of urban areas. Obtaining quantitative spatial information from the urban environment in fields such as determining the height of buildings plays an essential role in urban planning, monitoring damage to buildings, establishing communication bases and digital cities. During the last two decades, the use of Tomosar approach in order to reconstruct the structures of... 

Aircraft Range extension is a very common modification for today’s aircraft. Using the conformal fuel tanks is an appropriate method for today’s military and civil aircraft. In this thesis, a Design method has been developed for designing these tanks using DSM, CFD and DOE. The main goal in developing the method was to be able to consider these tanks as an add-on feature for the aircraft, which can be designed after the aircraft production. The main aspects of designing the tanks was to identify the consequences of adding the tanks to the aircraft. Due to limited resources, only the Longitudinal static and dynamic stability of the aircraft has been considered.The method introduced here shows... 

The main subject of this research is to investigate the effects of two parallel wingtip vortices on each other. In fact, the goal is to determine how far apart the two runways should be when two aircraft take off from parallel runways at the same time so that the effects of the two wingtip vortices on adjacent aircraft are minimized. This research study is important to prevent the occurrence of tragic accidents caused by the failure to observe safe separation between aircraft. It is believed that the failure to observe the minimum separation and the subsequent occurrence of accidents are due to the significant increase in the volume of air traffic in the world. For this reason, the...